Pros
It's a "fun" place to work at, with still a lot of impact that can be done. The environment feels young and people who work at Headspace are really nice to work with. If you're a "people person" you may love this place, but not so much if you want a place to build new and better.
Cons
It is a dead end for professional growth. Although if titles is what you're looking for, this place might have that for you. Otherwise, there's little opportunity for learning as ideas are heavily politicized. The organization is something between small and big companies: they try to document everything but they don't want to document anything, they try to implement best practices but they don't want best practices, etc... it feels as if they just want to keep it a free and open playground but at the same time know they could be doing better. This ends up in an awkward place where you (as a developer) will feel restricted by internal operations and also lack some crucial direction.